Utilizing a collaborative study, patients with symmetrical diabetic retinopathy fulfilling criteria as outlined in the protocol will be treated with either Xenon Arc Photocoagulation, Argon Laser Photocoagulation or a combination of both. The eye to be treated will be randomly selected by the statistical center in Maryland, the changes in the ocular fundus will be coded by the reading center in Wisconsin. The ultimate purpose is to determine if photocoagulation is beneficial or not in the treatment of diabetic retinopathy.